This invention relates to a semiconductor device that includes at least one MOS transistor that is formed on a semiconductor substrate, and to a structure for protecting circuit elements such as transistors from events such as a surge input of excessive static electricity from the outside and from static electricity that is generated during fabrication.
As protection against external surge inputs such as static electricity, protection circuits were configured combining a variety of resistances, such as diffusion resistance and polysilicon resistance, or combining diodes and transistors between the bonding pads and internal circuitry.
In recent years, miniaturization of transistors has progressed to achieve higher operating speeds and higher component densities. One transistor structure is an LDD structure in which a low concentration diffusion region is formed on the gate end of the drain diffusion layer, or a double diffusion structure in which a low concentration region is formed by using the difference between the diffusion coefficients of arsenic and of phosphorous. These structures are used extensively for transistors with channel lengths of about one micron to achieve protection against hot carriers. In addition, the thickness of the gate film has become thinner with the miniaturization of transistors. For example, in the case of 0.5 micron design rule transistors, the silicon dioxide films used as the gate films have a thickness of 10 nm.
In this manner, as gate insulating film thickness has been progressively reduced along with insulating miniaturization of the circuit elements, the gate breakdown voltage has, of course, decreased. In the case of a 10-nm-gate insulating film in a 0.5 micron design rule transistor, the breakdown voltage is about 10 volts.
In order to configure an integrated circuit, a sufficient amount of protection circuits have to be created in order to protect the internal circuitry from excessive voltage surges having causes such as static electricity from the outside. In addition, it is necessary to have a structure that allows protection from static electricity that is generated by the IC fabrication process.
In the protective circuits of the prior art, diodes or transistors were used. However, the breakdown voltage of the circuit elements that form these protective circuits does not decrease much even if the transistors have been miniaturized. For example, the drain breakdown voltage of a 0.5-micron design rule transistor is about 13 volts, which is higher than that of the gate insulating film. The problem is that the protective circuit does not function as a protective circuit because before it acts, the gate insulating film is permanently destroyed.